In general, a material absorbs one photon having an energy corresponding to excitation energy but does not absorb a photon having an energy which is not within this energy. However, when the intensity of light beam is very strong, two photons the sum of energy of which corresponds to excitation energy can be absorbed at the same time (non-resonant two-photon absorption). The utilization of this nature makes it possible to cause photoreaction only in the vicinity of focus at which light is focused by lens. Thus, an arbitrary position in a space can be selected to develop excited state.
As applications utilizing excited state obtained by non-resonant two-photon absorption there are known three-dimensional optical recording, two-photon microscopic imaging, optical medicare (photodynamic therapy: PDT) and two-photon microfabrication. In particular, two-photon microfabrication makes the best use of the advantage that the spatial resolution of non-resonant two-photon absorption is very high to cause polymerization reaction in a very small space, making it possible to prepare a minute three-dimensional structure.
However, non-resonant two-photon absorption can occur very difficultly. The two-photon absorption cross-section indicating the easiness of two-photon absorption is normally as very small as several GM (GM=1×10−50 cm4s molecule−1 photon−1). Accordingly, all various applications utilizing non-resonant two-photon absorption have an extremely low sensitivity and thus require a high power laser. This greatly impedes the application of non-resonant two-photon absorption.
In recent years, compounds having a relatively large non-resonant two-photon absorption cross-section have been reported. Examples of two-photon microfabrication using these compounds are described in B. H. Cumpston et al., “Nature”, 1999, 398, 51, K. D. Belfield et al., “J. Phys. Org. Chem.”, 2000, 13, 837, etc.
However, these examples are disadvantageous in that since usable lasers have a wavelength range as narrow as from 730 nm to 800 nm, efficient two-photon polymerization can be effected only with extremely limited two-photon absorption materials adapted for the wavelength range. Further, most of the compounds exemplified in these references can be difficultly synthesized and have a poor stability.
As a recording medium for recording data with laser beam there has heretofore been known an optical disc such as CD-R and CD-RW. These optical discs employ a laser having a wavelength of about 780 nm. With the recent rapid development of data engineering, there has been a growing demand for recording media having a high capacity and density. In order to realize higher capacity and recording density, it is effective to reduce the diameter of laser beam for data recording as much as possible. However, the diameter of laser beam cannot be reduced beyond diffraction limit. It has been theoretically known that diffraction limit depends on the wavelength of laser beam and the shorter the wavelength of laser beam is, the smaller is diffraction limit. Thus, an optical disc which allows data recording with a laser beam having a wavelength shorter than 780 nm, which has been heretofore used, has been under development. For example, optical discs called DVD-R and DVD-RW have been proposed. DVD-R and DVD-RW employ a laser having a wavelength of from 600 nm to 700 nm to allow data recording at a higher capacity and density than CD-R and CD-RW. However, the laser technology is on a level such that the wavelength of laser beam has been reduced to around 600 nm at last. It will take much time to further spread short wavelength laser and optimize the constitution of recording media required therefore. To cope with this problem, the utilization of two (multi)-photon absorption process, which is one of non-linear optical effects, has been proposed as a means for obtaining a high capacity and density data recording medium.
Two-photon absorption is a phenomenon that a molecule absorbs two photon simultaneously to undergo excitation. Since such a molecule absorbs an energy as great as twice that of photon corresponding to the wavelength of laser beam with which it is irradiated, it can be excited even by a light beam having a longer wavelength free of linear absorption. Further, since the probability of occurrence of two-photon absorption is proportional to second power of intensity of light with which a material is irradiated, the distribution of intensity of laser beam inducing two-photon absorption has a width as narrow as half-width. This corresponds to further reduction of the diameter of laser beam, which makes it possible to recording data in a radius range smaller than the radium of laser beam with which the material is irradiated. Further, from the three-dimensional standpoint of view, two-photon absorption is induced only in a minute region having a strong laser beam intensity at the focal point of laser beam condensed by lens. Thus, no two-photon absorption occurs at any point falling outside the focus, making it possible to induce selective two-photon absorption in an arbitrary minute space in a three-dimensional space. In other words, data recording and reproducing can be made even in the depth direction of a three-dimensional space. Due to this nature, the utilization of two-photon absorption allows data recording at higher density without using a short wavelength laser in principle.
However, two-photon absorption is a non-linear optical process having an extremely small efficiency. Thus, there is little or no materials which perform efficiently two-photon absorption (compounds having a large two-photon absorption cross-section).
In recent years, some compounds were reported to be organic compounds having a great two-photon absorption cross-section as in B. A. Reinhardt et al., “Chem. Mater. 1998”, 10, 1863, M. Albota et al., “Science 1998”, 281, 1653, J. D. Bhawalkar et al., “Opt. Commun. 1996”, 124, 33, G. S. He et al., “Appl. Phys. Lett. 1995”, 67, 3703, G. S. He et al., “Appl. Phys. Lett. 1995”, 67, 2433, P. N. Pradad et al., “Nonlinear Optics 1999”, 21, 39, G. S. He et al., “Appl. Phys. Lett. 1996”, 68, 3549, G. S. He et al., “J. Appl. Phys. 1997”, 81, 2529, L. -Z. Wu et al., “Chem. Phys. Lett. 1999”, 315, 379, S. -J. Chung et al., “J. Phys. Chem. B 1999”, 103, 10741, G. S. He et al., “Opt. Lett. 1995”, 20, 435, and J. W. Perry et al., “Nonlinear Optics”, 1999, 21, 225.
These papers contain compounds having a two-photon absorption cross-section several times to thousands of times that of the conventional compounds. However, none of recording materials comprising these compounds have even been applied.
Further, non-linear optical effect is a non-linear optical response proportional to second, third or higher power of photoelectric field applied. As second order non-linear optical effects proportional to second power of photoelectric field applied there are known second harmonic generation (SHG), photorectification, photorefractive effect, Pockelse effect, parametric amplification, parametric oscillation, optical sum-frequency generation, optical difference frequency generation, etc. As third order non-linear optical effects proportional to third power of photoelectric field applied there are known third harmonic generation (THG), optical Kerr effect, self-focusing, self-defocusing, two-photon absorption, etc.
As non-linear optical materials exerting these non-linear optical effects there have been found numeral inorganic materials. However, such inorganic non-linear optical materials can be very difficultly put to practical use because they can be difficultly molecularly designed for desired non-linear optical properties or optimization of physical properties required for production of element. On the other hand, organic non-linear optical materials can be molecularly designed to not only optimize desired non-linear optical properties but also control other physical properties and thus have been noted as favorable non-linear optical materials which can be fairly put to practical use.
In recent years, among the non-linear optical properties of organic compounds, third order non-linear optical effects have been noted. Among these third order non-linear optical effects, non-resonant two-photon absorption and non-resonant two-photon absorption induced emission have been noted. Two-photon absorption is a phenomenon that a compound absorbs two photons simultaneously to undergo excitation. A phenomenon that two photons are absorbed in an energy region free of (linear) absorption band of compound is called non-resonant two-photon absorption. The non-resonant two-photon absorption induced emission is light emission which occurs in radiative relaxation process followed by non-resonant two-photon absorption.
Now, hereafter, the terms “two-photon absorption” and “two-photon emission” means “non-resonant two-photon absorption” and “non-resonant two-photon absorption induced emission”, even if not defined clearly.
The efficiency of non-resonant two-photon absorption is proportional to the square of photoelectric field applied (quadratic dependency on the incident intensity). Thus, when a two-dimensional plane is irradiated with laser beam, the absorption of two-photon occurs only at a position having a high electric field intensity in the central position in the laser spot while no absorption of two-photon occurs at a position having a low electric field intensity out of focus. In a three-dimensional space, on the other hand, two-photon absorption occurs only in a region having a high electric field intensity at the focus obtained by condensing laser beam through lens while no two-photon absorption occurs in other regions falling outside the focus because their electric field intensity is low. As compared with linear absorption in which excitation occurs at all positions in proportion to the intensity of photoelectric field applied, non-resonant two-photon absorption involves excitation only at one point in a space due to the aforementioned second power dependency characteristics and thus provides a remarkably improved spatial resolution. In general, in the case where non-resonant two-photon absorption is induced, a near infrared short pulse laser having a wavelength longer than the wavelength region where the (linear) absorption band of a compound is present and free of absorption is often used. Since a so-called transparent near infrared light free of (linear) absorption band of a compound is used, the excited light can reach the interior of the sample without being absorbed or scattered, making it possible to excite the interior of the sample at one point at an extremely high spatial resolution due to the second power dependency characteristics of two-photon absorption. Thus, non-resonant two-photon absorption and non-resonant two-photon absorption induced emission have been expected for application to two-photon imaging or two-photon photodynamic therapy (PDT) of living tissue. Further, since the use of non-resonant two-photon absorption or non-resonant two-photon absorption induced emission makes it possible to withdraw photons having a higher energy than that of incident photons, studies have been reported of upconversion lasing from the standpoint of wavelength conversion devices.
As an organic compound which performs efficiently two-photon absorption induced light emission or upconversion lasing there is known a so-called stilbazolium derivative (He, G. S. et al., “Appl. Phys. Lett. 1995”, 67, 3703, He, G. S. et al., “Appl. Phys. Lett. 1995”, 67, 2433, He, G. S. et al., “Appl. Phys. Lett. 1996”, 68, 3549, He, G. S. et al., “Appl. Phys. Lett. 1997”, 81, 2529, Prasad, P. N. et al., “Nonlinear Optics 1999”, 21, 39, Ren, Y. et al., “J. Mater. Chem. 2000”, 10, 2025, Zhou, G. et al., “Jpn. J. Appl. Phys. 2001, 40, 1250). Examples of application using two-photon absorption phenomenon of stilbazolium derivative having the specific structure are described in International Patent Publication WO9709043.
In the case where non-resonant two-photon absorption induced (light) emission is applied to imaging of living tissue, photodynamic therapy, upconversion lasing, optical limiting, etc., it is necessary that the organic compound have a high two-photon absorption induced emission efficiency (two-photon absorption cross-section) and a high two-photon absorption induced emission efficiency from the excitation state occurred by two-photon absorption. In order to obtain a two-photon absorption induced emission intensity as high as twice that of an organic compound, an excited light intensity as high as four times the conventional value is needed to give second power dependent characteristics of two-photon absorption. However, when a living tissue, for example, is irradiated with an excessively strong laser beam, it can be damaged by light or the two-photon light-emitting dye itself undergoes deterioration by light to disadvantage. Accordingly, in order to perform strong two-photon absorption induced (light) emission at a low excitation light intensity, it is necessary that an organic compound which performs two-photon absorption and two-photon absorption induced emission efficiently be developed. The two-photon emission efficiency of stilbazolium derivatives has not yet satisfied a level to be desired in actual application.